[Laurajean1005]

A year ago I changed the transfer case and motor. Purchased a used one from a junk yard. The car, upon accelerating, makes a thumping noise as it switches into gear. Thud, thud thud.

Mechanic took a look and said it’s either the transfer case (which we can disconnect until next winter), the differential or the transmission.

Also, I do not know the last time he changed the transmission oil and filter, so I asked if we should do that and he said, no with the mileage on the car, it would do more harm.

2007 X3 automatic, 170k miles.

Any thoughts?

[chassis]

Always change oil in lubricated mechanical devices. Like a transfer case or transmission. New oil should have ben used when the junkyard items were installed.